In an air-fuel ratio control apparatus of an internal combustion engine, a fuel injection quantity is generally computed based on a sucked air flow quantity detected by an air flow meter and an engine revolution number detected from an ignition signal of an ignition coil, and the computed value is appropriately corrected and an electromagnetic fuel injection valve is driven to supply an optimum quantity of fuel to the engine.
More specifically, in a low-speed/low-load to medium-speed/medium-load driving region, an actual air-fuel ratio is detected from the oxygen concentration in the exhaust gas by an oxygen sensor attached to an exhaust manifold, and the above-mentioned fuel injection quantity is corrected and the feedback control is performed so that the air-fuel ratio is controlled to the theoretical air-fuel ratio (target air-fuel ratio).
Furthermore, at the time of start, idle driving or low water temperature driving or in a predetermined high-speed/high-load driving region, the feedback control of the air-fuel ratio is stopped, but the air-fuel ratio is feed-forward-controlled to the target air-fuel ratio.
If a misfire takes place in the engine, an unburnt fuel is discharged into an exhaust passage to cause deterioration of a catalyst, and especially, if a misfire continues for several seconds in the high-speed/high-load driving region, it is apprehended that the catalyst temperature will rise above 900.degree. C. In this state, damage to the catalyst is brought about.
According to the automobile standards instituted in the state of California, U.S.A., it is stipulated that when a trouble (mishap or deterioration) occurs in parts such as a fuel injection valve, an oxygen sensor and a catalyst device, if the trouble does not appear in the exhaust gas or drivability, this trouble should be made known in some form, though it is not necessary when a user can grasp the trouble as a phenomenon.
Under this background, the present invention has been completed, and it is an object of the present invention to provide a method an apparatus for judging a misfire with high accuracy without disposing any particular sensor even if the misfire does not appear in the exhaust gas or drivability.